Field of the Invention
The present invention relates to an imaging lens which forms an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device, and more particularly to an imaging lens which is built in an image pickup device mounted in an increasingly compact and low-profile mobile terminal such as a smartphone, mobile phone, tablet or PDA (Personal Digital Assistant), or a game console, or an information terminal such as a PC, or a highly functional product such as a home appliance with a camera function.
Description of the Related Art
In recent years, there has been a general tendency that many information terminals have a camera function. Also, home appliances with a camera are becoming widely used. For example, by telecommunication between a home appliance and a smartphone, a user away from home can monitor in real time what is going on at home or check how his/her child or pet is at home or control the home appliance to optimize its operation. Furthermore, wearable devices, such as glasses with a camera function and wrist watches with a camera function, have appeared in the market. It is thought that a variety of high value-added products which enhance consumer convenience and consumer satisfaction will be increasingly developed in the future by adding a camera function to various existing products. The cameras mounted in such products are required not only to provide high resolution to cope with an increase in the number of pixels but also to be compact and low-profile and offer high brightness and a wide field of view.
However, in order to provide a low-profile imaging lens with high brightness and a wide field of view as described above, the problem with difficulty in correction of aberrations in the peripheral area of an image has to be addressed, and unless the problem is solved, it will be difficult to deliver high imaging performance throughout the image.
With the recent trend toward image sensors which deal with an increasing number of pixels, an imaging lens composed of seven constituent lenses is expected to properly correct such aberrations in the peripheral area of an image that cannot be corrected properly by an imaging lens composed of six constituent lenses and also thought to have potential to correct various aberrations properly and achieve compactness, a small F-value and a wide field of view in a balanced manner. For example, the imaging lens described in JP-A-2012-155223 (Patent Document 1) is known as such an imaging lens composed of seven constituent lenses.
Patent Document 1 discloses an imaging lens which includes, in order from an object side, a first biconvex lens, a second biconcave lens cemented with the first lens, a third negative meniscus lens having a convex surface on the object side, a fourth positive meniscus lens having a concave surface on the object side, a fifth negative meniscus lens having a convex surface on the object side, a sixth biconvex lens, and a seventh biconcave lens. In this imaging lens, the ratio between the focal length of the front lens group composed of the first to fourth lenses and the focal length of the back lens group composed of the fifth to seventh lenses is kept within a prescribed range so that the optical system is compact and various aberrations are corrected properly.
The imaging lens composed of seven constituent lenses as described in Patent Document 1 corrects various aberrations properly and offers relatively high brightness with an F-value from 2.09 to 2.35 and a relatively wide field of view of 33 degrees. However, the total track length is longer than the diagonal length of the effective imaging plane of the image sensor, so that it is difficult to apply the imaging lens to a device which is strongly expected to be compact and low-profile. If this lens system is designed to offer a wider field of view and higher brightness, it would be unable to correct aberrations in the peripheral area properly. Consequently, it would be difficult to reduce image artifacts throughout the image effectively and ensure high image quality. Furthermore, the manufacture of a cemented lens requires high precision manufacturing techniques because it involves troublesome alignment and lamination steps, so that it is difficult to mass-produce the imaging lens at low cost with high productivity.
As mentioned above, in the conventional art, it is difficult to provide a low-profile low-cost imaging lens which offers high brightness, high resolution and a wide field of view.